Facile One-Pot Synthesis of Polydopamine Carbon Dots for Photothermal Therapy

Carbon dots (CDs) are a member of fluorescent carbon nanomaterials that are widely applied in bioimaging, photothermal therapy (PTT), and biosensors for its tunable fluorescence, photothermal conversion property, and excellent biocompatibility. Surface passivation and doping especially the doping of N atoms are critical factors to enhance the fluorescent intensity of CDs. Until now, a variety of nitrogen-rich molecules has been applied for the surface passivation of CDs such as L-Dopa, amino acids, and polyethylenimine (PEI). Herein, we report the synthesis of fluorescent polydopamine (PDA)-passivated carbon dots (CD-PDA) via one-pot microwave-assisted pyrolysis within 5 min, dramatically simplifying the reaction process compared with the hydrothermal treatment reported before. DLS, FT-IR, UV-Vis, and fluorescence spectroscopy were used to confirm the components of CD-PDA and to illuminate the mechanism of its tunable photoluminescence (PL). Due to the doping of N atoms by PDA, quantum yield (QY) of the CD-PDA was measured at 5%, which was nearly triple the original CDs without adding PDA. Yield of CD-PDA was about 1.5 times of the CDs on account of the enhancement of nucleation site for the carbon dot formation with the phenolic group provided by PDA. Meanwhile, photothermal conversion efficiency of the CD-PDA was determined to be 35% because of the excellent NIR light-thermal conversion property of PDA. Overall, we provided an extremely efficient approach to fabricate the fluorescent N-doped CD-PDA with stable photothermal conversion efficiency and excellent biocompatibility. More importantly, the passivation of PDA enabled the CD-PDA synthesized in our research compatible for further modification through Michael addition or Schiff base reaction.